This invention relates generally to electronic components, and more particularly concerns trimmable chip stubs.
The electronics industry is continually called upon to make products smaller and more powerful. Applications such as mobile phones, portable computers, computer accessories, hand-held electronics, etc., create a large demand for smaller electrical components. These applications further drive technology to research new areas and ideas with respect to miniaturizing electronics and using less expensive materials. Unfortunately, the technology is often limited due to the inability to make certain components smaller, more powerful, or less expensive. Nowhere can this be seen more than in the struggle to manufacture small and inexpensive printed circuit board (PCB) components.
Originally, electrical components were mounted on a PCB by inserting the leads of the component through the PCB and soldering them to solder pads on the opposite side of the PCB, (called through-hole technology). This technique left half of the PCB unpopulated because one side had to be reserved for solder pads and solder. Therefore, in order to fit more components in a particular circuit, the PCBs were made larger, or additional PCBs were required. Such configurations required more PCB material and space. All of which, had the effect of increasing costs.
The solution to this problem came in the form of Surface-Mount Devices (SMD), or Surface-Mount Technology. SMDs allow electrical components to be mounted on one side of a PCB, (i.e., without requiring the device leads to be inserted through holes). An SMD device has small solder pads (leads or terminations) connected to its body, which correspond to solder pads or lands placed on the surface of the PCB. Typically the PCB is run through a solder-paste machine (or screen printer), which puts a small amount of solder on the solder pads or lands of the PCB. Next, a glue dot is inserted on the PCB where the component is to rest. Then the component is placed on the PCB (temporarily held by the glue dot), and the PCB is sent through a re-flow oven to heat the solder paste and solder the component leads to the PCB solder pads or lands (thereby electrically connecting the component to the rest of the circuit). The components are generally supplied in a tape and reel carrier and are extracted by a robotic arm or movable chuck having a vacuum nozzle that removes the component from its carrier and places it on the PCB. As such, it is beneficial to have flat components to aid in the vacuum assembly""s ability to remove the component from its packaging. In addition, it is favorable to have non-polarized parts so that the component can be placed on the board in any direction, or in whatever direction the reel is set up to feed the component.
The primary advantage to SMD components is that both sides of the PCB can now be populated by electronic components, which means that one PCB today can roughly accommodate the same amount of electrical components which generally required two PCBs in the past. This technology has eliminated a significant amount of cost incurred by manufacturers, and greatly reduced the amount of space needed for electronic circuits.
As a result of this advancement in technology, current electronic circuit costs are mainly dictated by the costs of the individual components used on the PCB, the height requirement of each component, and the amount of time it takes to manufacture the finished product. In other words, if the electronic components are made less expensive, take up less space, and make for a more efficient process of manufacturing, the overall circuit will be less expensive and more compact. Unfortunately, there is a trade-off in making certain electrical components less costly because the desired parameters for the component cannot be achieved when using less expensive materials, or there is an inherent limitation in making the component less costly because it cannot be manufactured using less expensive materials. Trimmable chip stubs (or trimmable chip capacitors) are good examples of this.
Capacitors are circuit elements having two conducting surfaces or plates separated by a nonconducting material, or dielectric. Positive charges are transferred to one plate and negative charges to the other. The charge differential between the conductive surfaces or plates creates an electric field that stores energy. Because of the presence of the nonconducting dielectric, the conduction current that flows in the wires connecting the capacitor to the circuit cannot flow internally between the conductive surfaces or plates. However, due to the presence of electromagnetic fields, a displacement current, which is approximately equal to the conduction current, flows between the plates of the capacitor. Certain parameters of these components are affected by the type of material used to manufacture the component. For instance, the type of material used for the dielectric determines the dielectric constant and cost of the component. As the dielectric constant is increased, the capacitance is increased. For general applications in electronic circuits, the dielectric material may be air, Mylar, polystyrene, mica, glass, ceramics, etc. Typically, ceramics are used because of their high dielectric constant (which allows for large capacitance-to-volume ratios).
In a trimmable chip capacitor, at least one of the conductive surfaces or plates of the capacitor can be trimmed to alter the capacitance of the component, without causing a severe reduction in the quality factor, or Q, of the component. The quality factor is the ratio of the capacitor""s reactance to its resistance at a specified frequency. The capacitor plate can be trimmed in several different ways. Prototypes are often trimmed by use of drill or milling machine. However, components that are being mass produced are typically laser trimmed. The advantage to using trimmable chip components is that the component and/or circuit can be tuned to fall within a desired range of operation. For instance, each component has a tolerance within which it is specified to perform. Given the various acceptable values of operation within the tolerance range, and the number of various components used in a circuit, the operation of the circuit as a whole is likely to differ slightly circuit to circuit. By allowing one of the components to be trimmed after completion of the circuit, a specified range of performance for the component and/or the entire circuit can be reached simply by trimming the component until it or the circuit falls within the desired range.
A multi-layer trimmable chip capacitor is disclosed in U.S. Pat. No. 5,264,983, to Petrinec. The component of Petrinec is constructed with spaced apart electrodes embedded in a double layered dielectric body connected to exterior ends on the body. Another electrode in the form of a conductor plate lies on top of the body and is trimmable to alter the capacitance of the component without significantly altering its Q. Petrinec""s trimmable chip capacitor is a high loss part that uses a large amount of power, has a low Q, and thus is believed to be incapable of operating properly during high power applications, (i.e., radio frequency/microwave applications). More particularly, Petrinec""s trimmable chip capacitor may not be able to handle the high displacement currents associated with high power applications, which would result in the component heating up. Since capacitance and frequency are affected by changes in temperature, the component may not perform within its specified parameters, and in a worst case scenario may heat to temperatures that can damage the circuit and/or break the components electrical connection with the rest of the circuit. Further, the Petrinec component has a fairly complex and highly specialized construction as described above that makes manufacture expensive and time consuming as the components are individually produced via the customized layering process described by Petrinec.
Another problem that has been identified with existing multi-layer tunable chip capacitors is conductor power metalization loss. Multi-layer ceramic capacitors are formed by making a sandwich of conductor coated ceramic layers. Due to their construction and the extremely high temperatures used during the firing process in the fabrication of these multi-layer ceramic capacitors, the conductor metalization can become oxidized thereby increasing the power loss of the component.
A further problem with existing multi-layer tunable chip capacitors is dielectric loss. In the fabrication of multi-layer chip capacitors, different formulations of filling materials (or fillers) are used to make ceramic into a fabrication xe2x80x9ctapexe2x80x9d for creating the required formation of a multi-layer ceramic dielectric. These fillers have the effect of increasing the loss characteristic of the ceramic material, thereby contributing to the overall power loss of the component.
An additional problem with existing multi-layer tunable chip capacitors is resonance loss. Current multi-layer chip capacitors can have a large amount of parasitic inductance due to the architecture of the component. This parasitic inductance and some conductor metalization and dielectric losses, when combined with the capacitance of the structure, leads to parallel resonance loss over some frequency band of operation. In fact, the current multi-layer capacitor structures will decrease in Q as the component is tuned, as by trimming.
Presently, high power trimmable capacitors are constructed out of ceramic substrates and graphite carriers, and are fabricated directly on the substrate material. The cost of these materials makes the end product more expensive to manufacture and reduces the number of available suppliers. In addition, the necessity to fabricate the component on the substrate material increases costs due to difficulty in manufacturing and increased scrap product. For example, if an error occurs during the trimming of a component fabricated directly on the substrate, the entire circuit must be scrapped, or a costly and time consuming xe2x80x9cpaintingxe2x80x9d process must be undertaken in which the component is reconstructed and re-trimmed.
Other attempts have been made to laser tune circuit stubs fabricated on fiberglass PCBs instead of ceramic substrates, however failures have occurred due to the high laser power levels required to remove conductive stub materials. Specifically, the high power levels associated with the laser cause the PCB material to burn or carbonize. As with the use of trimmable capacitive components fabricated on ceramic substrates, once a mistake has been made trimming, the entire board must be scrapped, thereby increasing the cost of production. In addition to causing carbonization, the laser frequently removes more than just the stub conductor material. For example, the laser often cuts through the PCB material, jeopardizing the ability to populate both sides of the PCB and/or risking the accidental severing or damaging of traces on the PCB.
Accordingly, there is a need for an improved trimmable chip stub which is less expensive and is a low loss part capable of operating within defined parameters during high power applications. Further a trimmable chip stub that is capable of being easily and economically replaced if problems occur in tuning the component is desired.